"IFR" means Instrument Flight Rules. This refers to regulations, not conditions of flight. Flying under IFR does not suggest that one is in conditions that make flight by reference to instruments necessary, or that one is in instrument conditions.
Accordingly, there is little point in logging "IFR time."
One should log instrument time. However, simply because one is operating under IFR does not imply that one is entitled to log instrument time...because a great deal of IFR flying isn't in instrument conditions. Conversely, one may log instrument time under VFR...the rules (IFR vs. VFR) have nothing at all to do with logging instrument time, but the conditions of flight have a great deal to do with logging of instrument time.
Being above FL180 does not suggest anything about the logging of flight time, nor should one log instrument time merely because one is operating above FL180. One should only log instrument time when one is in conditions that require flight instruments to safely operate the airplane. These conditions may be simulated or actual instrument conditions external to the cockpit. If one is ot in these conditions, one should not log instrument time.